This invention relates to microwave communication systems, and particularly to a digital phase modulator for use in the transmitter portion of a microwave communication system.
The formation of a constant envelope type phase modulated digital signal (BPSK or QPSK) for a microwave satellite communication system involves translating a digitally modulated signal through a succession of up converters which are separated by amplifiers and spectrum shaping filters, followed by a high power amplifier and sometimes a microwave bandpass filter, the signal then being transmitted (typically at 6 Gigahertz or 14 Gigahertz) to an earth satellite. As it is very difficult to realize a microwave filter with a bandwidth of less than about 0.1% without excessive filter loss, spectrum shaping for low capacity terminals, (i.e. having a relatively narrow bandwidth, rarely exceeding about 35 kilohertz for each voice circuit), is usually performed just following the first intermediate frequency amplifier, immediately following the modulator, or at baseband. Following spectrum shaping, the signal must pass only through linear components to the antenna. If non-linear components are encountered, additional sidebands are generated, causing interference between adjacent channels.
Since the active components, and particularly the high power amplifier are increasingly non-linear as their power handling capacity is increasingly utilized, the signals passing therethrough must be kept to considerably lower power levels than the high power amplifier would otherwise be able to handle. For example, where a travelling wave tube amplifier is employed as the high power amplifier, it must be used in its linear mode, e.g. typically at 1/5th of its maximum power handling level. Clearly this is wasteful and costly since the equipment is under-utilized.